The invention relates to micro-electromechanical (MEM) micromirror type optical switches, in particular to the testing of the operation of such switches.
Optical communication systems require high speed data, implemented as optical signals, to be switched between ports of a switching device to allow a signal routing function. Typically, the optical signals are carried by optical fibers, which connect to the optical switching device. There are currently a number of methods for achieving the required switching operation.
One solution comprises an electromechanical arrangement, where a signal in an optical fiber A is routed to fiber B by mechanically aligning fiber A with fiber B. This arrangement is bulky and mostly suited only to 1xc3x97N switch configurations.
An alternative solution is to use a hybrid optical switch in which the optical signals are first converted to electrical signals which are switched in a conventional manner. The resulting outputs of the switch are then converted back to optical signals. This adds complexity and noise to the switching operation.
Optical switches are also known in which a control signal is used to vary the path of an optical signal. For example, waveguide-based switches rely on the change of refractive indices in the waveguides under the influence of an external electric field, current or other signal.
Optical switches using an array of mirrors which can be mechanically tilted are also known. Small micromirrors (for example less than 1 mm) are arranged in a line or array, and the incident light signal is deflected by controlling the tilt angle of each micromirror. Mirror type optical switches include digital micromirror devices which tilt each micromirror by electrostatic force, piezoelectric drive micromirror devices which tilt each micromirror by a fine piezoelectric element and electromagnetic devices which rely upon electromagnetic and electrostatic forces.
In a typical micromirror device, a plurality of micromirrors are arranged in an array of Nxc3x97M mirrors. Each micromirror can be controlled and is capable of switching between a first reflection state and a second non-reflection state. The optical signal is routed between an input and a selected output by controlling the reflection state of each mirror.
It has been recognised that testing of the operation of the micromirrors is desirable. U.S. Pat. No. 5,796,508 discloses a method for testing micromirrors of an array. The described method requires a test signal and test sensor to be connected to the array in place of the normal signal inputs and outputs. The mirrors are controlled in turn, and the test circuitry then determines whether or not the required routing operations are implemented. A problem with this technique is that the test circuitry must be coupled to the normal input and outputs of the device. This may not be possible after the optical switch is installed within a system. Furthermore, a test signal must be routed to each input of the switch, which increases the amount of required testing circuitry. It is also desirable to test the mirror array before completion of manufacture, for example before packaging of the array which involves providing the signal input and output connections. The prior art method uses the normal signal inputs and outputs and therefore requires a fully packaged array.
According to the first aspect of the invention there is provided an optical switch comprising a switching array of micromirrors having a plurality of inputs and a plurality of outputs, the micromirrors of the switching array each having first and second positions. A first test array of mirrors has a test input and a plurality of outputs, the first test array enabling a test signal to be routed to mirrors of the switching array. A second test array of mirrors has a test output, the test input of the first test array being routed to the test output when the mirror being tested is in one of the first and second positions.
The first test array enables a single test signal to be routed to a selected input of the optical switch, without disrupting the normal signal connections to the optical switch. This enables the optical switch to be tested easily before or after installation, and enables the optical switch to be fully tested using a single test signal input and output.
The outputs of the first test array may be aligned optically with the inputs to the switching array. Thus, the first test array comprises mirrors which are additional to the mirrors of the optical switch, and located at the input to the optical switch. The mirrors of the test array may, however, be formed on the same substrate and with the same processing steps as the mirrors of the optical switch.
Alternatively, the first test array can comprise mirrors of the switching array and comprise double-sided mirrors, the first test array then enabling a test signal to be routed to the remaining mirrors of the switching array.
The first test array further comprises a plurality of signal inputs. In normal operation (i.e. not during testing) the first test array is transparent to the input signals of the optical switch, so that normal operation of the switch is not affected by the first test array.
The first and second test arrays may be arranged on opposite sides or adjacent sides of the switching array. Depending on the configuration, correct operation of the mirror being tested may be determined either when the test signal reaches the test output, or else when the test signal is prevented from reaching the test output.
The invention also provides an optical switching system comprising:
a plurality of optical switches of the invention;
an input micromirror array having a combined test input and a plurality of outputs, each output being aligned optically with an input to the first test array of a respective one of the optical switches, thereby enabling the combined test input to be routed to the first test array of each optical switch; and
an output micromirror array having a plurality of inputs from the second test arrays of each optical switch and a combined test output.
This system enables multiple optical switches within a system to be tested using a single test signal, which can be routed in turn to the optical switches.
The optical switch of the invention can be used in an optical node, for example comprising a demultiplexing unit receiving a group of WDM channels and for providing individual channels on individual optical fibers, the optical switch, a multiplexing unit having as inputs the outputs of the optical switch, the multiplexing unit combining the individual channels into a single WDM signal on an individual optical fiber.
According to a second aspect of the invention, there is provided a method of testing a mirror within a micromirror array optical switch, the method comprising:
applying a test signal to a first test array of mirrors;
positioning a mirror of the first test array in a predetermined position, thereby routing the test signal to a selected input of the optical switch, the mirror to be tested being associated with the selected input;
positioning the mirror to be tested of the optical switch in a testing position;
receiving a test output from a second test array of mirrors, the test signal being routed to the test output or being intercepted from being routed to the test output when the mirror being tested is in the testing position.
This testing method uses a single test signal applied to a single input of the first test array to enable the optical switch to be tested. The method can be adapted to enable multiple optical switches to be tested. In particular, a combined test signal can be provided to an input micromirror array which selectively routes the test input to a first test array of mirrors of a selected optical switch. The test output from the selected optical switch is provided to an output micromirror array which routes the test output from the selected optical switch to a combined test output.
When the optical switch of the invention is provided in an optical node, the test input and output enable testing of not only the correct operation of the mirrors in the optical switch, but also testing of the correct routing of channels through the node. The invention also provides methods of testing the connection of an optical switch within an optical communications system node, particularly for testing the routing through multiplexers and demultiplexers of the node.
The invention also provides an optical switch comprising a switching array of micromirrors having a plurality of inputs and a plurality of outputs, the micromirrors of the switching array each having first and second positions; and a test array of mirrors having a test input and a plurality of outputs, the test array enabling a test signal to be routed to mirrors of the switching array. This arrangement enables a test signal to be supplied into a network at the optical switch, without disrupting the normal signal connections within the network.
The invention also provides an optical switch comprising a switching array of micromirrors having a plurality of inputs and a plurality of outputs, the micromirrors of the switching array each having first and second positions; and a test array of mirrors having a number of inputs connected to or optically aligned with the outputs of the switching array, and a test output. This arrangement enables a test signal to be tapped out of the optical switch of a network for monitoring, without disrupting the normal signal connections within the network.